The invention relates to improvements in paper making machines, and more particularly improvements in rolls for a press section wherein the roll has a superior wetting characteristic for the easy and gentle release of a wet web having a low wet strength.
In paper making machines a web is typically formed in a forming section and is carried on through a press section, then to a dryer section and then to a calender. In the press section, the wet web passes over rolls, in some instances supported by felts and in other instances, directly in contact with the surface of the roll so that on the offrunning side of the roll, the web must be separated from the roll surface. The tension created in the web by being pulled off of the roll surface is critical, and it is essential, particularly for handling and transferring a lightweight low wet strength ground wood sheet that the roll readily release the sheet. To attain this operation without danger of tearing the sheet, the roll must of necessity have good wetting characteristics so that a film of water is always present on the roll surface to insure sheet release. For this purpose, it has heretofore been customary to provide a granite roll with a granite outer surface to achieve this good wetting characteristic. An example of such a granite roll is shown in the Hill patent, U.S. Pat. No. 3,737,962. Disadvantages accompany the provision of granite rolls in that such rolls are more expensive to manufacture than the conventional metal rolls. Further, granite rolls do not have the bending strength of metal rolls and in very wide machines, do bend more than desirable. The construction and maintenance of granite rolls is more expensive than metal rolls, and the difficulties encountered with providing an annular granite shell for a granite roll are indicated in the aforementioned U.S. Pat. No. 3,737,962.
In the manufacture of conventional large metal press rolls, a process which has been adapted is generally known as centrifugal casting, and an example of such a process is shown in the Justus et al U.S. Pat. No. 3,414,044 wherein a multilayered roll is shown with a chill iron outer layer.
I have discovered that a roll with superior wetting characteristics is provided by making a roll by the centrifugal casting process wherein the outer layer is of iron with substantially 25% chromium content. This roll attains the strength characteristics of a conventional cast iron roll and is relatively inexpensive to manufacture as contrasted with a granite roll, and avoids the disadvantages of low strength and expense accompanying a granite roll. Yet, the chrome containing outer layer has superior wetting characteristics so as to provide good release of a web by maintaining a continual layer of water on the outer surface for release of a wet web, and particularly for the easy release of a low wet strength lightweight ground wood sheet.
For manufacture of this roll, a centrifugal annular mold is provided and a first ladle is poured into the roll of substantially 25% chrome iron. A second ladle of cast iron is poured and the roll is formed by centrifugal casting. The chrome iron roll has the same desirable wetting characteristics as a granite roll. The chrome containing roll is also corrosion resistant, and it has a hard surface for better wear resistance from doctors. The versatility of the chrome containing roll is greater than a granite roll. For example, such a roll can be manufactured as a controlled crown roll wherein the roll can be supported or bent to achieve certain crowning effects or to compensate for deflection due to nip pressures in a manner not possible with a granite roll.
Accordingly, an object of the present invention is to provide a roll for a press section in a paper making machine having an outer surface with good wetting characteristics which is less expensive to manufacture and to maintain than a granite surfaced roll.
A further object of the invention is to provide an improved press roll which avoids the disadvantage present with a granite roll and provides better surface wear and better deflection characteristics than a granite roll.
Other objects, advantages and features, as well as equivalent structures and methods, will become more apparent with the teaching of the principles of the present invention is connection with the disclosure of the preferred embodiment in the specification, claims and drawings, in which: